It is known in compact disc players and similar articles for light from a laser diode to be focussed on a particular spot of the disc and the reflected light directed back through the optical system to a detector. The amount of information contained on a disc is great in comparison to its size and thus the beam of laser light must be focussed very accurately onto a particular point, so that the detector is not confused by signals reflected from other points on the disc. This has been achieved in the past by means of a somewhat complex series of lenses which are cumbersome to move so as to view the entire surface of the disc. Alternatives have included a single lens to focus the light from the laser diode directly onto this point of the disc. This however suffers from the disadvantage that the spacing of the diode and the information on the disc must be accurately known.
It is an object of the present invention to provide a lens system which overcomes the above disadvantages. It is a further object of the lens to correct for axial and radial spherical aberration using one, e.g. the first, surface, and for coma by using the other. (Field curvature is less important since very small off-axis angles are used.) It is a further object to provide a method of making a plastic lens for use in such a lens system.
One problem which may be encountered in producing such accurate lenses is lack of register between the two dies used in injection moulding the lens. So called centration errors may occur when the centres of curvature of the two faces are not exactly aligned. A decentration of 2 .mu.m may be sufficient to destroy the diffraction limited performance of the lens. This is because an error in one face will cause large changes in the angles of incidence of light rays at an opposite face of the lens.
Similarly, errors in the thickness of the lens of as little as 5 .mu.m may have a very similar effect for much the same reasons.
It has now been found that optical problems due to centration errors or errors in thickness may be obviated if the surfaces of the lens are so adapted that light always passes within the lens as a parallel beam. This means, of course, that each surface must be extremely accurately configured as an aspheric curve so that light arriving at the edges of the lens is diffracted to be parallel to light arriving at or near the centre of the lens. However, since the angle of incidence of light emerging from the lens is always the same, whatever the relative position of the surfaces, much larger centration errors or variations in thickness do not cause the same problems.